The present invention relates to a safety device for an electric power tool, especially a portable tool, with a main switch, electronic controls, a tachometer-generator, and a relay that electrically parallels the motor, that acts on the supply of current to the motor through a relay contact, and that is activated through the tachometer-generator and through a subsequent safety circuit.
Known electric power tools are turned on and off through a main switch. The motor is governed by electronic controls that obtain an actual value from a tachometer-generator. When the controls break down, the motor can run too fast. The speed can increase to the extent that, for example, the grinding disk on an angled grinder can burst. It is accordingly desirable for the motor to switch off when the controls break down.
One means of ensuring that they will do so is a centrifugal-force switch of the type described in German Pat. No. 638 928. The mechanical components involved in this mode of ensuring that the motor will switch off at excessively high speeds make it very expensive, however. The device is also very sensitive to contamination, from the aspect of accuracy of function for example.